Hydraulic system for hydraulically actuating an ambulance lifting table

ABSTRACT

A hydraulic system for hydraulically actuating a vertically adjustable ambulance lifting table used for receiving thereon a stretcher and adapted to be selectively switched to a cushioned mode of operation comprises a cylinder for actuating the lifting table, a hydraulic accumulator for cushioning the lifting table, a pump, a three-way valve, and a discharge valve connected to a reservoir of the hydraulic system on the outlet side thereof. In order to prevent the patient and/or the medical personnel from being exposed to danger by the lifting table when the above-mentioned hydraulic system is used, and even if the hydraulic system is actuated incorrectly, the three-way valve is in fluid connection with the pump and the hydraulic accumulator on the inlet side thereof and with the cylinder on the outlet side thereof, a valves is provided between the cylinder on the one hand and the hydraulic accumulator on the other, the valves is controlled such that it assumes an open position only if the pressure in the hydraulic accumulator exceeds the pressure within the cylinder by not more than a predetermined pressure difference, and the input side of the discharge valve is in fluid connection either with the hydraulic accumulator or with the cylinder depending on the pressure prevailing in the hydraulic accumulator relative to the pressure prevailing in the cylinder.

FIELD OF THE INVENTION

The present invention refers to a hydraulic system for hydraulicallyactuating an ambulance lifting table. In particular, the presentinvention deals with a hydraulic system for hydraulically actuating avertically adjustable ambulance lifting table used for receiving thereona stretcher and adapted to be selectively switched to a cushioned modeof operation, said hydraulic system comprising a cylinder for actuatingthe lifting table, a hydraulic accumulator for cushioning the liftingtable, a pump, a three-way valve, and a discharge valve means connectedto a reservoir on the outlet side thereof.

DESCRIPTION OF THE PRIOR ART

It has been known for a fairly long time to hydraulically actuatehydraulically the lifting table which is used for receiving thereon astretcher in an ambulance. The hydraulic system for hydraulicallyactuating the lifting table must fulfill a plurality of requirements soas to permit treatment of a patient at a so-called shock position, atwhich the table is secured in position at its top or bottom dead centre,within the ambulance, or so as to obtain a cushioned support of thetable at a central position of the cylinder. The requirements which haveto be fulfilled by such a hydraulic system include, as will be shown bythe explanations following hereinbelow, also strict safetyspecifications which are intended to prevent the patient from beingharmed even if the hydraulic system for the hydraulic actuation of thelifting table is actuated incorrectly.

In the known hydraulic system of the type mentioned at the beginning,the hydraulic accumulator is connected directly to the cylinder and canbe blocked by a pressure-independent 2/2-way valve. This known hydraulicsystem can be operated incorrectly in different ways or it may happenthat technical defects occur, which may cause danger to the patient andthe medical personnel in the ambulance. It is, for example, imaginablethat the patient who first lies on the stretcher is very heavy and that,when the patient has been moved, the lifting table is lowered down toits bottom dead centre prior to removing the stretcher from the liftingtable. If, during this lowering operation, the residual pressure is notfully discharged because of a technical defect, e.g. because of anelectrical disconnection, a high pressure corresponding to the heavyweight of said patient will still prevail in the hydraulic accumulator.If the next patient who is placed on the lifting table on a stretcher isnot heavy and if the fluid connection to the lifting cylinder is openedby pressing the "lift" button, the high pressure, which is still storedin the hydraulic accumulator, will cause the lifting table to moveabruptly upwards. It need not be explained in detail that, especially inthe case of patients with back injuries, jerky operations of the liftingtable must not be permitted under any circumstances. For eliminatingthis kind of problem, it has already been suggested to provide thehydraulic accumulator with a manometer so that the ambulance attendantor some other operator can recognize a dangerous high pressure in thehydraulic accumulator. Taking into account the hectic kind of daily workwhich has to be done by emergency medical personnel when they are onduty, it cannot be expected that the emergency medical personnel willactually check the manometer display prior to each actuation of thelifting table.

A dangerous pressure difference between the hydraulic accumulator andthe lifting cylinder will also be generated if a patient or an operatorleaves the table in the blocked condition of the cushioning system. Ifsome type of adjusting knob is now actuated, the table will inevitablymove upwards abruptly. This movement can be so violent that acute dangermay even be caused to persons who stand or sit in the vicinity of thetable and who touch the table with a part of their body. If, due to atechnical defect, the table is discharged by a hand-operated emergencydischarge valve, it depends on the operator how long he maintains saidhand-operated emergency discharge valve open, when the table has arrivedat its lower position, so as to reduce the residual pressure in thehydraulic accumulator, a process which may require up to 10 seconds'time. If the table with the stretcher is now removed from the vehicleand the patient is unloaded, the table will either move abruptly upwardsduring the unloading operation or the next patient who is less heavythan the preceding one will be catapulted upwards, depending on whetherthe hydraulic accumulator is non-current-carrying and blocked or open.

DE-A-2538411 already discloses a hydraulic system used for hydraulicallyactuating an ambulance lifting table and comprising a cylinder foractuating the lifting table, a hydraulic accumulator in the form of anexpansion tank, a pump and a valve means. A sharp-off valve is providedbetween the hydraulic accumulator and the cylinder, for arresting thecushioning of the ambulance lifting table; also provided is a non-returnvalve that is arranged in parallel with a plate orifice. This knownsystem also has the disadvantages and risks discussed hereinbefore. Ifthe shut-off valve is blocked in a condition in which a high load isapplied to the ambulance lifting table, a high pressure will continue toexist in the hydraulic accumulator. If the shut-off valve is re-openedin this condition for initiating the cushioned mode of operation of theambulance lifting table, the ambulance lifting table will carry out anabrupt upward movement. This abrupt upward movement of the table mayinvolve danger to patients and to the ambulance attendants, which hasbeen explained hereinbefore.

SUMMARY OF THE INVENTION

Taking this prior art as a basis, it is therefore the object of thepresent invention to further develop the hydraulic system, which is usedfor hydraulically actuating an ambulance lifting table and which hasbeen mentioned at the beginning, in such a way as to reduce danger tothe patient and/or the medical personnel by the movement of the liftingtable, if the hydraulic system is actuated incorrectly.

This object is achieved by a hydraulic system for hydraulicallyactuating a vertically adjustable ambulance lifting table used forreceiving thereon a stretcher and adapted to be selectively switched toa cushioned mode of operation, said hydraulic system comprising:

a cylinder for actuating the lifting table,

a hydraulic accumulator for cushioning the lifting table,

a pump,

a three-way valve in fluid connection with the pump and the hydraulicaccumulator on the inlet side thereof and with the cylinder on theoutlet side thereof, the three-way valve limiting the amount of fluidthat can be supplied to the cylinder,

a discharge valve means connected to a reservoir of the hydraulic systemon the outlet side thereof, the discharge valve means being connected tothe hydraulic accumulator on its input side, and

a valve means provided between the cylinder on the one hand and thehydraulic accumulator on the other, said valve means being controllablesuch that it assumes an open position only if the pressure in thehydraulic accumulator exceeds the pressure within the cylinder by notmore than a predetermined pressure difference.

SHORT DESCRIPTION OF THE DRAWINGS

In the following, a preferred embodiment of the hydraulic systemaccording to the present invention will be explained in detail withreference to the drawing enclosed, in which:

FIG. 1 shows a connection diagram of the hydraulic system according tothe preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As can be seen in the FIG. 1, the hydraulic system according to thepresent invention, which serves to hydraulically actuate a verticallyadjustable ambulance lifting table H used for receiving thereon astretcher (not shown) and adapted to be selectively switched to acushioned mode of operation, comprises a cylinder Z for actuating saidlifting table H, a hydraulic accumulator B for cushioning the liftingtable, a pump P for supplying the hydraulic fluid, and a hydrauliccontrol system, which is arranged between these components of the systemand a reservoir T and which is designated generally by reference numeralST.

In a first branch Z1 of the hydraulic system, which extends between thehydraulic accumulator B and the cylinder Z, an orifice plate BD ispositioned, which is used for producing a pressure drop depending on theflow rate of the fluid flowing to the cylinder Z, a three-way-valve Dfor limiting the amount of fluid which can be supplied to the cylinder Zto e.g. approx. 0.7 l/min, and a first non-return valve R1, which can beopened only in the direction of the cylinder Z and which isspring-loaded by means of a spring F (not shown) in a direction oppositeto its opening direction so that it can only be opened when the pressureprevailing in the hydraulic accumulator B is above the cylinder pressureof approx. 1 bar. The pressure difference determined by this spring F isadapted to the pressure loss occurring in the hydraulic accumulator B,when the pressure in said hydraulic accumulator has been raised to aspecific operating pressure starting from 0 pressure, whereby the gas isheated in the hydraulic accumulator B, the subsequent cooling down ofsaid gas causing the pressure difference which is to be compensated forby said spring F.

In a second branch Z2, which is parallel to said first branch Z1 andwhich extends between the cylinder Z and the hydraulic accumulator B, avalve means is located, which comprises a second valve V2 connected inseries with a third valve V3, said valves being not pressurecompensated. The third valve V3 permits small leakage in bothdirections.

Between said first branch Z1 and said second branch Z2, a first valve V1is located.

For reasons which will be explained hereinbelow, the valve V2 isconstructed such that it can only be moved to its open position if thepressure applied to its connection facing the hydraulic accumulatorexceeds the pressure applied to its connection facing away from thehydraulic accumulator by not more than a predeterminable pressuredifference. The operating advantages which can thus be achieved will bediscussed in connection with the explanation of the function of thesystem.

Also the valve V3 can only be opened if the pressure in the cylinder Zexceeds the pressure on the side of the hydraulic accumulator by notmore than a predetermined difference.

The hydraulic series connection of a non-return valve R3, a fourth valveV4 and a two-way controller V5, which limits the discharge flow rate ofthe hydraulic flow from the hydraulic accumulator B into the reservoirT, is located in a third branch Z3 extending between the hydraulicaccumulator B and the reservoir T. The fourth valve V4 is connected to acommon junction of a third and a fourth non-return valve R3, R4 so thatthe respective higher pressure in the hydraulic accumulator B and thecylinder Z, respectively, can first be reduced.

The third and the fourth non-return valve R3, R4, each of which can onlybe opened in the direction of a hand-operated emergency valve HN, arelocated between the hydraulic accumulator B on the one hand or thecylinder Z on the one hand and said hand-operated emergency valve HN onthe other. The outlet of the hand-operated emergency valve HN isconnected to the inlet of the two-way valve V5.

Finally, a fifth non-return valve R5, which can be opened only in thedirection of the hydraulic accumulator B, is located between the pump Pand the hydraulic accumulator B. A fixedly set pressure-limiting valve Ais located between the pump P and the reservoir T.

In the following, the mode of operation of the system shown in thefigure will be explained. The valves are shown in the figure in anon-current-carrying condition.

Taking as a basis an initial position of the lifting table H at thebottom dead centre thereof, the pump P as well as the first valve V1 andthe second valve V2 have a current supplied thereto for lifting thelifting table. Initially, the pump P pumps the fluid into the hydraulicaccumulator. When the pressure required for lifting the cylinder Z hasbeen reached, the cylinder will be accelerated continuously and slowlydepending on the total flow resistance in branch Z1 until it has reachedits maximum speed depending on the properties of the three-way valve D.In the course of this process, the pressure in the hydraulic accumulatorexceeds the pressure in the cylinder Z by an amount corresponding to theflow resistance in branch Z1. The amount of fluid delivered by the pumpP must be larger than the maximum amount of fluid flowing through thethree-way valve D. It follows that a constant lifting movement iscarried out until the pump P is switched off. The excessive amountdelivered will be discharged in the three-way valve D into the reservoirT.

A short time before the desired level of the lifting table cylinder Zhas been reached, the pump is switched off. A short time afterwards, thefirst valve V1 is switched off. The second valve V2 is a valve which isnot pressure compensated and which, starting from its closed position,cannot be opened as long as the pressure in the hydraulic accumulatorexceeds the pressure in the lifting table cylinder by a predeterminedrelative differential pressure amount.

Exactly the opposite is the case for the valve V3. If the pressure inthe cylinder Z is higher than the pressure on the side of the hydraulicaccumulator, said valve V3 cannot be opened. Hence, the table is alsoprevented from falling down.

When the lifting table has been pumped to the desired position, viz.e.g. to the cushioned central position, the pump P will be switched off.In this situation, a continuous deceleration of the lifting table H isachieved by the decreasing amount of fluid flowing through the orificeplate BD and the three-way valve D until said lifting table stops at itsend position. It follows that, instead of jerky switching off, thelifting movement fades away continuously. When the table has come to astandstill, the first valve V1 is closed.

When the lifting table H has been raised to its central position andcome to an standstill, current can be supplied to the two valves V2 andV3 so that the table can yield. If the table has to be locked inposition, e.g. for reviving a patient, the second and third valves V2,V3 are switched off simultaneously. In this locked condition, which isdetermined by the second and third valves V2, V3, it may happen that theload acting on the lifting table H suddenly decreases, if, for example,a person leaves the lifting table H or if a patient is picked up fromsaid lifting table H. In order to prevent abrupt rising or lowering ofthe table in response to an activation of the cushioning which may nowoccur, the valve V3 has the property of leaking. This has the effectthat the actual pressure in the cylinder Z is reported to the valve V2.Said valve V2 will only open if the pressure is approximately balanced.Uncontrolled upward movements of the table will thus be avoided. If thepressure on the side of the cylinder is higher, the third valve V3 willnot open so that abrupt dropping of the table will be prevented.

For lowering the lifting table H, the valves V2, V3 and V4 have currentsupplied thereto. The table starts to move downwards with a pleasantslow movement, if there is no substantial difference between thepressures in the cylinder and in the hydraulic accumulator. If thepressure is higher on the cylinder side, the cylinder Z will first belowered whereupon the hydraulic accumulator B will be emptied, since,when the lowermost position has been reached, the valve V4 has stillcurrent supplied thereto for a period of approx. 10 seconds. If thepressure in the hydraulic accumulator B is higher, fluid will first bedischarged from said hydraulic accumulator. Also if the pressures areequal, the cylinder will be lowered. When the central position isreached during the table lowering movement from the uppermost position,it is first only the valve V4 that is closed, whereas the connection viathe valves V2 and V3 is still kept open for a short period of time sothat the table will be stopped in a manner which is agreeable to thepatient.

If, e.g. due to the fact that a patient has been removed from thelifting table H, the hydraulic accumulator pressure should, in thiscondition, be higher than the pressure of the cylinder Z, the hydraulicaccumulator will first be emptied via the fourth valve V4 and thetwo-way valve V5 until the pressure in the lifting cylinder is equal tothe pressure in the hydraulic accumulator so that the fluid can flow viathe valves V2 and V3 and so that also the lifting cylinder will belowered.

When the hand-operated emergency valve is actuated, also this will havethe effect that the pressure is first reduced in that area of thehydraulic accumulator B and of the lifting cylinder Z, respectively,which has the higher pressure at the moment in question, until pressureadaptation has been effected.

According to an essential aspect of the present invention, the secondand third valves V2, V3 can be connected in series as far as the supplyof current is concerned. In other words, this aspect according to thepresent invention discloses that the second and third valves V2, V3 areprovided with solenoids M2, M3, which are electrically connected inseries, for their actuation. This has the effect that the powerconsumption of the current taken from the vehicle battery is reduced bythree quarters in the condition in which the cushioning is switched on.Due to the low power consumption of the second and third valves V2, V3,which is achieved by the series connection, it is here also possible touse versions which are open when no current is supplied, since powerconsumption can then only take place in the closed condition of thesecond and third valves V2, V3, which causes the cushioned mode ofoperation of the lifting table H.

The non-return valves R3, R4 are spring-loaded. Preferably, the springpreload of the fourth non-return valve R4 exceeds the spring preload ofthe third non-return valve R3 in such a way that the fourth non-returnvalve R4 opens at a pressure exceeding the opening pressure of the thirdnon-return valve R3 by 0.1 to 0.5 bar.

I claim:
 1. A hydraulic system for hydraulically actuating a verticallyadjustable ambulance lifting table used for receiving thereon astretcher and adapted to be selectively switched to a cushioned mode ofoperation, said hydraulic system comprising:a cylinder for actuating thelifting table, a hydraulic accumulator for cushioning the lifting table,a pump, a three-way valve in fluid connection with the pump and thehydraulic accumulator on the inlet side thereof and with the cylinder onthe outlet side thereof, the three-way valve limiting the amount offluid that can be supplied to the cylinder, a discharge valve meansconnected to a reservoir of the hydraulic system on the outlet sidethereof, the discharge valve means being connected to the hydraulicaccumulator on its input side, and a valve means provided between thecylinder on the one hand and the hydraulic accumulator on the other,said valve means being controllable such that it assumes an openposition only if the pressure in the hydraulic accumulator exceeds thepressure within the cylinder by not more than a predetermined pressuredifference.
 2. A hydraulic system according to claim 1, wherein theinput side of the discharge valve means is additionally adapted to beconnected to the cylinder.
 3. A hydraulic system according to claim 1,wherein the input side of the discharge valve means is in fluidconnection either with the hydraulic accumulator or with the cylinderdepending on the pressure prevailing in the hydraulic accumulatorrelative to the pressure prevailing in the cylinder.
 4. A hydraulicsystem according to claim 1, further comprising a first non-return valvearranged in series with the three-way valve in such a way that it canonly be opened for a flow of fluid in the direction of the cylinder. 5.A hydraulic system according to claim 1, further comprising an orificeplate which is connected in series with the three-way valve.
 6. Ahydraulic system according to claim 1, further comprising a first valvewhich is connected between the hydraulic accumulator and the three-wayvalve.
 7. A hydraulic system according to claim 1, wherein the valvemeans comprises a second valve connected in series with a third valve,said valves being not pressure compensated, and the third valve permitssmall leakage in both directions.
 8. A hydraulic system according toclaim 7, wherein the second and third valves are provided with solenoidswhich are electrically connected in series.
 9. A hydraulic systemaccording to claim 1, wherein the discharge valve means comprises afourth valve and a two-way valve, which is connected in series with saidfourth valve, for determining a discharge flow amount.
 10. A hydraulicsystem according to claim 9, further comprising a hand-operatedemergency valve whose input side is connected to the cylinder via afourth non-return valve and to the hydraulic accumulator via a thirdnon-return valve and whose output side is connected to the input of thetwo-way valve.
 11. A hydraulic system according to claim 9, wherein thefourth valve is connected to a common junction of the third and fourthnon-return valves so that the respective higher pressure in thehydraulic accumulator and the cylinder, respectively, can first bereduced.
 12. A hydraulic system according claim 11, wherein the fourthnon-return valve has a spring preload which exceeds a spring preload ofthe third non-return valve in such a way that the fourth non-returnvalve opens at a pressure exceeding the opening pressure of the thirdnon-return valve by 0.1 to 0.5 bar.